The production of turbine blades by a process in which a ceramic core is surrounded by a wax layer which may be injected or sprayed thereon and in which the wax layer, by repeated immersion and coating with ceramic material, is provided with a ceramic shell is known. After removal of the wax, the ceramic shell is fired and the space formed by the removal of the wax between the ceramic shell and the ceramic core is filled with a molten metal and, after hardening of the molten metal, the shell and the core are removed and the resulting metallic turbine blade blank can be mechanically processed, e.g. by machining.
Stabilization of the core within the shell is a problem.
In the past the lower end of the core, corresponding to the base of the blade, was fixed on casting. Nevertheless the free end of the blade, i.e. the end remote from the base, had a tendency to shift in the shell and cause variations in the wall thickness of the blade in the regions of the free end.
It has been proposed heretofore to press into the wax layer, upon which the shell is later formed, pins which rest against the core and can project beyond the wax layer (see DE 38 13 287 C2 and EP 0 324 229 B1). The ends of the pins projecting beyond the wax layer are embedded in the shell which is formed around the wax layer and then constitutes the mold for the cast metal. Such pins, after removal of the wax, tend to limit movement of the core. However, the fabrication of turbine blades by this method has been found to be expensive and this is in part because the setting of the pins is less than reliable. Furthermore, depending upon the pin material used, there may be local variations in material properties of the turbine blade which can give rise to problems, for example, in the subsequent coating of the turbine blades.
In another known process (see EP 0 585 183 A1) the core is provided with projections bumps which serve as spacers. It has also been proposed to remove the wax layer on the core at selected regions so that projections or spacers can be formed by the mold shell. This is described, for example, in U.S. Pat. No. 6,364,001. Even these methods are expensive from a production point of view, since they complicate the subsequent machining and do not always guarantee a reproducible, desired and reliable wall thickness at least at the free end of the turbine blade.